Tire pressure monitoring systems (TPMS) on vehicles are generally required in the U.S., with Europe and countries in Asia to follow. The legislation mandating the use of TPMS typically sets a pressure warning threshold level which is monitored by wheel-based units, or wheel modules, in direct TPMS. The wheel modules are mounted inside of each tire, such as on the rim, valve, or in-tire, in order to periodically or continuously monitor the inflation pressure of the tire.
Each wheel module typically includes a pressure sensor, control logic such as a microcontroller, a power source such as a battery, and a radio frequency (RF) transmitter that communicates pressure readings from the pressure sensor to a central TPMS receiver mounted elsewhere in the vehicle. Some wheel modules also comprise an acceleration sensor for determining when the vehicle is in motion in order to conserve battery life.
The process of identifying which wheel module sent a particular signal, and therefore which tire may have low pressure, is called localization. When a low pressure situation is detected, drivers generally want to know which tire is low, rather than simply that one of the tires is low, which often requires each to be checked in order to determine which tire actually needs attention. Effective and efficient localization is an on-going challenge in TPMS because tires are frequently rotated and sometimes changed out between summer and winter, altering their positions. Additionally, power constraints on the wheel modules make frequent communications and localization signal transmissions impractical.
In an example localization scheme utilizing an acceleration sensor, the radial component of the acceleration is determined in order to ascertain the direction of rotation of each wheel. However, depending on the location of the sensor in the respective tires, very high radial accelerations can be sensed. For example, for a vehicle with a 16-inch rim traveling at 200 km/h, an acceleration of 15,436 m/s2 can be sensed in the radial component. Therefore, difficulty exists in achieving the requisite resolution in the radial direction without the acceleration sensor spring constant becoming so small that the sensor mass strikes the walls of the sensor during normal operation.
Therefore, there is a need for improved localization techniques in TPMS systems utilizing acceleration sensors.